The emerging roles of β-arrestins in fibrotic diseases

β-Arrestin pathway activation by selective ATR1 agonism promotes calcium influx in podocytes, leading to glomerular damage

Angiotensin receptor blockers (ARBs) are the first-line treatment for hypertension; they act by inhibiting signaling through the angiotensin 1 receptor (AT1R). Recently, a novel biased AT1R agonist, TRV120027 (TRV), which selectively activates the β-arrestin cascade and blocks the G-protein-coupled receptor pathway has been proposed as a potential blood pressure medication. Here, we explored the effects of TRV and associated β-arrestin signaling in podocytes, essential cells of the kidney filter. We used human podocyte cell lines to determine β-arrestin's involvement in calcium signaling and cytoskeletal reorganization and Dahl SS rats to investigate the chronic effects of TRV administration on glomerular health. Our experiments indicate that the TRV-activated β-arrestin pathway promotes the rapid elevation of intracellular Ca2+ in a dose-dependent manner. Interestingly, the amplitude of β-arrestin-mediated Ca2+ influx was significantly higher than the response to similar Ang II concentrations. Single-channel analyses show rapid activation of transient receptor potential canonical (TRPC) channels following acute TRV application. Furthermore, the pharmacological blockade of TRPC6 significantly attenuated the β-arrestin-mediated Ca2+ influx. Additionally, prolonged activation of the β-arrestin pathway in podocytes resulted in pathological actin cytoskeleton rearrangements, higher apoptotic cell markers, and augmented glomerular damage. TRV-activated β-arrestin signaling in podocytes may promote TRPC6 channel-mediated Ca2+ influx, foot process effacement, and apoptosis, possibly leading to severe defects in glomerular filtration barrier integrity and kidney health. Under these circumstances, the potential therapeutic application of TRV for hypertension treatment requires further investigation to assess the balance of the benefits versus possible deleterious effects and off-target damage.

β-Blockers in Hepatosplenic Schistosomiasis: A Narrative Review

Hepatosplenic schistosomiasis (HSS) is a serious complication of chronic schistosomiasis that can result in portal hypertension and variceal bleeding. β-blockers, a class of medications commonly used to treat hypertension and other cardiovascular conditions, have been investigated for their potential use in preventing variceal bleeding in HSS. Several studies have shown that β-blockers can reduce portal pressure and prevent variceal bleeding effectively in these patients. However, there are limited data on the long-term efficacy and safety of β-blockers in this setting, and further research is needed to determine the optimal use of these medications. This review summarizes the evidence supporting current recommendations of β-blocker use in patients with HSS.

Hepatic β-arrestins: potential roles in liver health and disease

Β-arrestins are intracellular scaffolding proteins that have multifaceted roles in different types of disorders. In this review article, we gave a summary about the discovery, characterization and classification of these proteins and their intracellular functions. Moreover, this review article focused on the hepatic expression of β-arrestins and their hepatocellular distribution and function in each liver cell type. Also, we showed that β-arrestins are key regulators of distinct types of hepatic disorders. On the other hand, we addressed some important points that have never been studied before regarding the role of β-arrestins in certain types of hepatic disorders which needs more research efforts to cover.

GPR35 acts a dual role and therapeutic target in inflammation

GPR35 is a G protein-coupled receptor with notable involvement in modulating inflammatory responses. Although the precise role of GPR35 in inflammation is not yet fully understood, studies have suggested that it may have both pro- and anti-inflammatory effects depending on the specific cellular environment. Some studies have shown that GPR35 activation can stimulate the production of pro-inflammatory cytokines and facilitate the movement of immune cells towards inflammatory tissues or infected areas. Conversely, other investigations have suggested that GPR35 may possess anti-inflammatory properties in the gastrointestinal tract, liver and certain other tissues by curbing the generation of inflammatory mediators and endorsing the differentiation of regulatory T cells. The intricate role of GPR35 in inflammation underscores the requirement for more in-depth research to thoroughly comprehend its functional mechanisms and its potential significance as a therapeutic target for inflammatory diseases. The purpose of this review is to concurrently investigate the pro-inflammatory and anti-inflammatory roles of GPR35, thus illuminating both facets of this complex issue.

β-arrestin2 deficiency ameliorates S-100-induced autoimmune hepatitis in mice by inhibiting infiltration of monocyte-derived macrophage and attenuating hepatocyte apoptosis

Autoimmune hepatitis (AIH) is a progressive hepatitis syndrome characterized by high transaminase levels, interface hepatitis, hypergammaglobulinemia, and the presence of autoantibodies. Misdiagnosis or delayed treatment of AIH can lead to cirrhosis or liver failure, which poses a major risk to human health. β-Arrestin2, a key scaffold protein for intracellular signaling pathways, has been found to be involved in many autoimmune diseases such as Sjogren's syndrome and rheumatoid arthritis. However, whether β-arrestin2 plays a role in AIH remains unknown. In the present study, S-100-induced AIH was established in both wild-type mice and β-arrestin2 knockout (Arrb2 KO) mice, and the experiments identified that liver β-arrestin2 expression was gradually increased, and positively correlated to serum ANA, ALT and AST levels during AIH progression. Furthermore, β-arrestin2 deficiency ameliorated hepatic pathological damage, decreased serum autoantibody and inflammatory cytokine levels. β-arrestin2 deficiency also inhibited hepatocyte apoptosis and prevented the infiltration of monocyte-derived macrophages into the damaged liver. In vitro experiments revealed that β-arrestin2 knockdown suppressed the migration and differentiation of THP-1 cells, whereas β-arrestin2 overexpression promoted the migration of THP-1 cells, which was regulated by the activation of the ERK and p38 MAPK pathways. In addition, β-arrestin2 deficiency attenuated TNF-α-induced primary hepatocyte apoptosis by activating the Akt/GSK-3β pathway. These results suggest that β-arrestin2 deficiency ameliorates AIH by inhibiting the migration and differentiation of monocytes, decreasing the infiltration of monocyte-derived macrophages into the liver, thereby reducing inflammatory cytokines-induced hepatocytes apoptosis. Therefore, β-arrestin2 may act as an effective therapeutic target for AIH.

FGF9 Recruits β-Catenin to Increase Hepatic ECM Synthesis and Promote NASH-Driven HCC

Most nonalcoholic steatohepatitis (NASH) patients develop severe fibrosis through extracellular matrix (ECM) accumulation, which can lead to hepatocellular carcinoma (HCC). Fibroblast growth factor 9 (FGF9) is involved in serial types of cancer; however, the specific role of FGF9 in NASH-driven HCC is not fully understood. This study finds that FGF9 is increased in patients with NASH-associated HCC. Furthermore, NASH-driven HCC mice models by feeding wildtype mice with high-fat/high-cholesterol (HFHC) diet and low dose carbon tetrachloride (CCl4 ) treatment is established; and identified that hepatic FGF9 is increased; with severe fibrosis. Additionally, AAV-mediated knockdown of FGF9 reduced the hepatic tumor burden of NASH-driven HCC mice models. Hepatocyte-specific FGF9 transgenic mice (FGF9Alb ) fed with a HFHC diet without CCl4 treatment exhibited an increased hepatic ECM and tumor burden. However, XAV-939 treatment blocked ECM accumulation and NASH-driven HCC in FGF9Alb mice fed with HFHC diet. Molecular mechanism studies show that FGF9 stimulated the expression of ECM related genes in a β-catenin dependent manner; and FGF9 exerts its effect on β-catenin stability via the ERK1/2-GSK-3β signaling pathway. In summary, the data provides evidence for the critical role of FGF9 in NASH-driven HCC pathogenesis; wherein it promotes the tumors formation through the ECM pathway.

Beta-blockers in patients with liver cirrhosis: Pragmatism or perfection?

With increasing decompensation, hyperdynamic circulatory disturbance occurs in liver cirrhosis despite activation of vasoconstrictors. Here, the concept of a therapy with non-selective beta-blockers was established decades ago. They lower elevated portal pressure, protect against variceal hemorrhage, and may also have pleiotropic immunomodulatory effects. Recently, the beneficial effect of carvedilol, which blocks alpha and beta receptors, has been highlighted. Carvedilol leads to "biased-signaling" via recruitment of beta-arrestin. This effect and its consequences have not been sufficiently investigated in patients with liver cirrhosis. Also, a number of questions remain open regarding the expression of beta-receptors and its intracellular signaling and the respective consequences in the intra- and extrahepatic tissue compartments. Despite the undisputed role of non-selective beta-blockers in the treatment of liver cirrhosis, we still can improve the knowledge as to when and how beta-blockers should be used in which patients.

Targeting GPCRs to treat cardiac fibrosis

Cardiac fibrosis occurs ubiquitously in ischemic heart failure, genetic cardiomyopathies, diabetes mellitus, and aging. It triggers myocardial stiffness, which impairs cardiac function, ultimately progressing to end-stage heart failure and increased mortality. Although several targets for anti-fibrotic therapies have been identified, including TGF-β and receptor tyrosine kinase, there is currently no FDA-approved drug specifically targeting cardiac fibrosis. G protein-coupled receptors (GPCRs) are integral, multipass membrane-bound receptors that exhibit diverse and cell-specific expression, offering novel and unrealized therapeutic targets for cardiac fibrosis. This review highlights the emerging roles of several GPCRs and briefly explores their downstream pathways that are crucial in cardiac fibrosis. We will not only provide an overview of the GPCRs expressed on cardiac fibroblasts that are directly involved in myofibroblast activation but also describe those GPCRs which contribute to cardiac fibrosis via indirect crosstalk mechanisms. We also discuss the challenges of identifying novel effective therapies for cardiac fibrosis and offer strategies to circumvent these challenges.

Lysyl oxidase promotes renal fibrosis via accelerating collagen cross-link driving by β-arrestin/ERK/STAT3 pathway

Lysyl oxidase (LOX) is a copper‐dependent monoamine oxidase whose primary function is the covalent cross‐linking of collagen in the extracellular matrix (ECM). Evidence has shown that LOX is associated with cancer and some fibrotic conditions. We recently found that serum LOX is a potential diagnostic biomarker for renal fibrosis, but the mechanism by which LOX is regulated and contributes to renal fibrosis remains unknown. The current study demonstrates the following: (1) LOX expression was increased in fibrotic kidneys including ischemia‐reperfusion injury‐(IRI‐), unilateral ureteral obstruction‐(UUO‐), and folic acid‐ (FA‐) induced fibrotic kidneys as well as in the paraffin‐embedded sections of human kidneys from the patients with renal fibrosis. (2) The increasing deposition and cross‐linking of collagen induced by LOX was observed in IRI‐, UUO‐ and FA‐kidneys. (3) LOX was regulated by the β‐arrestin‐ERK‐STAT3 pathway in renal fibrosis. STAT3 was the downstream of AT1R‐β‐arrestin‐ERK, ERK entered the nucleus and activated STAT3‐pY705 but not STAT3‐pS727. (4) STAT3 nuclear subtranslocation and binding to the LOX promoter may be responsible for the upregulation of LOX expression. (5) Pharmacologic inhibition of LOX with BAPN in vivo inhibited the upregulation of LOX, decreased collagen over cross‐linking and ameliorated renal fibrosis after ischemic injury. Collectively, these observations suggest that LOX plays an essential role in the development of renal fibrosis by catalyzing collagen over cross‐linking. Thus, strategies targeting LOX could be a new avenue in developing therapeutics against renal fibrosis.

Research Progress of Fibroblast Growth Factor 21 in Fibrotic Diseases

Fibrosis is a common pathological outcome of chronic injuries, characterized by excessive deposition of extracellular matrix components in organs, as seen in most chronic inflammatory diseases. At present, there is an increasing tendency of the morbidity and mortality of diseases caused by fibrosis, but the treatment measures for fibrosis are still limited. Fibroblast growth factor 21 (FGF21) belongs to the FGF19 subfamily, which also has the name endocrine FGFs because of their endocrine manner. In recent years, it has been found that plasma FGF21 level is significantly correlated with fibrosis progression. Furthermore, there is evidence that FGF21 has a pronounced antifibrotic effect in a variety of fibrotic diseases. This review summarizes the biological effects of FGF21 and discusses what is currently known about this factor and fibrosis disease, highlighting emerging insights that warrant further research.

Sympathetic Denervation Ameliorates Renal Fibrosis via Inhibition of Cellular Senescence

Objective

Continuous overactivation of the renal sympathetic nerve is considered to be an important cause of renal fibrosis. Accumulated senescent cells in the damaged kidney have metabolic activities and secrete amounts of proinflammatory factors as part of the SASP (the senescence-associated secretory phenotype), which induce chronic inflammation and fibrosis. It is still unclear whether renal sympathetic nerves affect renal inflammation and fibrosis by regulating cellular senescence. Therefore, we hypothesize that sympathetic activation in the injured kidney induces cellular senescence, which contributes to progressive renal inflammation and fibrosis.

Methods

Renal denervation was performed 2 days before the UUO (unilateral ureteral obstruction) and UIRI (unilateral ischemia-reperfusion injury) models. The effects of renal denervation on renal fibrosis and cellular senescence were observed. In vitro, cellular senescence was induced in renal proximal tubular epithelial cell lines (TKPTS cells) by treatment with norepinephrine (NE). The selective α_2A-adrenergic receptor (α_2A-AR) antagonists BRL44408 and β-arrestin2 siRNA, were administered to inhibit NE-induced cellular senescence. A significantly altered pathway was identified through immunoblotting, immunofluorescence, immunocytochemistry, and functional assays involved in mitochondrial function.

Results

Renal fibrosis and cellular senescence were significantly increased in UUO and UIRI models, which were partially reversed by renal denervation. In vitro, NE induced epithelial cells secreting proinflammatory cytokines and promoted cell senescence by activating α_2A-AR. Importantly, the effects of NE during cellular senescence were blocked by α_2A-AR selective antagonist and β-arrestin2 (downstream of α_2A-AR) siRNA.

Conclusion

Renal sympathetic activation and cellular senescence are important neurometabolic and neuroimmune mechanisms in the development of renal fibrosis. Renal sympathetic neurotransmitter NE acting on the α_2A-AR of epithelial cells promotes cellular senescence through the downstream β-arrestin2 signaling, which is a potential preventive target for renal fibrosis.

β-arrestin2 regulating β2-adrenergic receptor signaling in hepatic stellate cells contributes to hepatocellular carcinoma progression

Background: β-arrestin2 and β2-adrenergic receptor (β2-AR) have important roles in malignant tumors, the present study aims to investigate the role of activated β2-AR in hepatic stellate cells (HSCs) during hepatocellular carcinoma (HCC) progression and the regulatory effect of β-arrestin2.

Methods: Immunofluorescence and Western blot were used to detect the expression of β-arrestin2 and β2-AR in HSCs of liver tissues from human HCC samples and diethylnitrosamine (DEN)-induced HCC model mice. We next used β-arrestin2^-/- mice to demonstrate the regulatory role of β-arrestin2 in DEN mice. The subsets of T cells were quantified by flow cytometry. MTT and wound healing assay were applied to detect the proliferation and migration of cells. Co-immunoprecipitation assay was used to detect the link of β-arrestin2 and β2-AR in HSCs. Effect of β-arrestin2 overexpression on β2-AR downstream signaling pathway was verified by Western blot. The secretion of CCL2 was detected by ELISA.

Results: The expression of β2-AR was significantly increased, while β-arrestin2 was decreased in HSCs of HCC tissues. And β-arrestin2 deficiency exacerbates DEN-induced HCC accompanied with increased β2-AR expression. The results of flow cytometry showed that the percentage of activated T cells decreased gradually after DEN injection. β-arrestin2 knockout down-regulated the ratio of activated T cells. In vitro, selective activation of β2-AR in HSCs promoted the proliferation and migration of HCC cells. β-arrestin2 overexpression enhanced co-immunoprecipitation of β-arrestin2 and β2-AR in activated HSCs, and decreased its downstream Akt phosphorylation. Akt inhibitor decreased secretion of CCL2 in activated HSCs.

Conclusion: Our study demonstrated that β2-AR activation in HSCs induces the proliferation and migration of HCC cells may be through Akt signaling, and this effect appears to be regulated by β-arrestin2.

β-Arrestin2 deficiency attenuates oxidative stress in mouse hepatic fibrosis through modulation of NOX4

Hepatic fibrosis is a disease characterized by excessive deposition of extracellular matrix (ECM) in the liver. Activation of hepatic stellate cells (HSCs) is responsible for most of ECM production. Oxidative stress and reactive oxygen species (ROS) may be important factors leading to liver fibrosis. NADPH oxidase 4 (NOX4) is the main source of ROS in hepatic fibrosis, but the mechanism by which NOX4 regulates oxidative stress is not fully understood. β-Arrestin2 is a multifunctional scaffold protein that regulates receptor endocytosis, signaling and trafficking. In this study, we investigated whether β-arrestin2 regulated oxidative stress in hepatic fibrosis. Both β-arrestin2 knockout (Arrb2 KO) mice and wild-type mice were intraperitoneally injected with carbon tetrachloride (CCl4) to induce hepatic fibrosis. Arrb2 KO mice showed significantly attenuated liver fibrosis, decreased ROS levels and NOX4 expression, and reduced collagen levels in their livers. In vitro, NOX4 knockdown significantly inhibited ROS production, and decreased expression of alpha-smooth muscle actin in angiotensin II-stimulated human HSC cell line LX-2. Through overexpression or depletion of β-arrestin2 in LX-2 cells, we revealed that decreased β-arrestin2 inhibited ROS levels and NOX4 expression, and reduced collagen production; it also inhibited activation of ERK and JNK signaling pathways. These results demonstrate that β-arrestin2 deficiency protects against liver fibrosis by downregulating ROS production through NOX4. This effect appears to be mediated by ERK and JNK signaling pathways. Thus, targeted inhibition of β-arrestin2 might reduce oxidative stress and inhibit the progression of liver fibrosis.

Inflammasome as an Effective Platform for Fibrosis Therapy

Fibrosis is the final stage of the development of chronic inflammation. It is characterized by excessive deposition of the extracellular matrix, leading to tissue structure damage and organ dysfunction, which is a serious threat to human health and life. However, the molecular mechanism of fibrosis is still unclear. Inflammasome is a molecular complex of proteins that has been becoming a key innate sensor for host immunity and is involved in pyroptosis, pathogen infection, metabolic syndrome, cellular stress, and tumor metastasis. Inflammasome signaling and downstream cytokine responses mediated by the inflammasome have been found to play an important role in fibrosis. The inflammasome regulates the secretion of IL-1β and IL-18, which are both critical for the process of fibrosis. Recently, researches on the function of inflammasome have attracted extensive attention, and data derived from these researches have increased our understanding of the effects and regulation of inflammasome during fibrosis. In this review, we emphasize the growing evidence for both indirect and direct effects of inflammasomes in triggering fibrosis as well as potential novel targets for antifibrotic therapies.

The role and mechanism of β-arrestin2 in signal transduction

β-arrestin2 is a ubiquitously expressed scaffold protein localized on the cytoplasm and plasma membrane. It was originally found to bind to GPCRs, uncoupling G proteins and receptors' binding and inhibiting the signal transduction of the GPCRs. Further investigations have revealed that β-arrestin2 not only mediates the desensitization of GPCRs but also serves as a multifunctional scaffold to mediate receptor internalization, kinase activation, and regulation of various signaling pathways, such as TLR4/NF-κB, MAPK, Wnt, TGF-β, and AMPK/mTOR pathways. β-arrestin2 regulates cell invasion, migration, autophagy, angiogenesis, and anti-inflammatory effects by regulating various signaling pathways, which play a vital role in many physiological and pathological processes. This paper reviews the structure and function of β-arrestin2, the regulation of β-arrestin2 based signaling pathways. The role and mechanism of β-arrestin2 signaling have been delineated in sufficient detail. The prospect of regulating the expression and activity of β-arrestin2 in multisystem diseases holds substantial therapeutic promise.

G protein-coupled receptors as potential targets for nonalcoholic fatty liver disease treatment

Nonalcoholic fatty liver disease (NAFLD) is a broad-spectrum disease, ranging from simple hepatic steatosis to nonalcoholic steatohepatitis, which can progress to cirrhosis and liver cancer. Abnormal hepatic lipid accumulation is the major manifestation of this disease, and lipotoxicity promotes NAFLD progression. In addition, intermediate metabolites such as succinate can stimulate the activation of hepatic stellate cells to produce extracellular matrix proteins, resulting in progression of NAFLD to fibrosis and even cirrhosis. G protein-coupled receptors (GPCRs) have been shown to play essential roles in metabolic disorders, such as NAFLD and obesity, through their function as receptors for bile acids and free fatty acids. In addition, GPCRs link gut microbiota-mediated connections in a variety of diseases, such as intestinal diseases, hepatic steatosis, diabetes, and cardiovascular diseases. The latest findings show that gut microbiota-derived acetate contributes to liver lipogenesis by converting dietary fructose into hepatic acetyl-CoA and fatty acids. GPCR agonists, including peptides and natural products like docosahexaenoic acid, have been applied to investigate their role in liver diseases. Therapies such as probiotics and GPCR agonists may be applied to modulate GPCR function to ameliorate liver metabolism syndrome. This review summarizes the current findings regarding the role of GPCRs in the development and progression of NAFLD and describes some preclinical and clinical studies of GPCR-mediated treatment. Overall, understanding GPCR-mediated signaling in liver disease may provide new therapeutic options for NAFLD.

Anti-ATR001 monoclonal antibody ameliorates atherosclerosis through beta-arrestin2 pathway

BACKGROUND

Our previous study developed ATRQβ-001 vaccine, which targets peptide ATR001 from angiotensin Ⅱ (Ang Ⅱ) receptor type 1 (AT1R). The ATRQβ-001 vaccine could induce the production of anti-ATR001 monoclonal antibody (McAb-ATR) and inhibit atherosclerosis without feedback activation of the renin-angiotensin system (RAS). This study aims at investigating the underexploited mechanisms of McAb-ATR in ameliorating atherosclerosis.

METHODS

AT1R-KO HEK293T cell lines were constructed to identify the specificity of McAb-ATR and key sites of ATRQβ-001 vaccine. Beta-arrestin1 knock-out (Arrb1^-/-) mice, Beta-arrestin2 knock-out (Arrb2^-/-) mice, and low-density lipoprotein receptor knock-out (LDLr^-/-) mice were used to detect potential signaling pathways affected by McAb-ATR. The role of McAb-ATR in beta-arrestin and G proteins (G_q or G_i^2/i3) signal transduction events was also investigated.

RESULTS

McAb-ATR could specifically bind to the Phe¹⁸²-His¹⁸³-Tyr¹⁸⁴ site of AT1R second extracellular loop (ECL2). The anti-atherosclerotic effect of McAb-ATR disappeared in LDLr^-/- mice transplanted with Arrb2^-/- mouse bone marrow (BM) and BM-derived macrophages (BMDMs) from Arrb2^-/- mice. Furthermore, McAb-ATR inhibited beta-arrestin2-dependent extracellular signal regulated kinase1/2 (ERK1/2) phosphorylation, and promoted beta-arrestin2-mediated nuclear factor kappa B p65 (NFκB p65) inactivity. Compared with conventional AT1R blockers (ARBs), McAb-ATR did not inhibit Ang Ⅱ-induced uncoupling of heterotrimeric G proteins (G_q or G_i^2/i3) and G_q-dependent intracellular Ca²⁺ release, nor cause RAS feedback activation.

CONCLUSIONS

Through regulating beta-arrestin2, McAb-ATR ameliorates atherosclerosis without affecting G_q or G_i^2/i3 pathways. Due to high selectivity for AT1R and biased interaction with beta-arrestin2, McAb-ATR could serve as a novel strategy for treating atherosclerosis.

Sodium butyrate protects against lipopolysaccharide-induced liver injury partially via the GPR43/ β-arrestin-2/NF-κB network

Background

Butyrate acts as a regulator in multiple inflammatory organ injuries. However, the role of butyrate in acute liver injury has not yet been fully explored. In the present study, we aimed to investigate the association between butyrate and lipopolysaccharide (LPS)-induced acute liver injury and the signaling pathways involved.

Methods

LPS-induced acute liver injury was induced by intraperitoneal injection of LPS (5 mg/kg) in G-protein-coupled receptor 43 (GPR43)-knockout (KO) and wild-type female C57BL/6 mice. Sodium butyrate (500mg/kg) was administered intraperitoneally 30 min prior to LPS exposure. Liver injury was detected by serum markers, tissue morphology, and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Pro-inflammatory-factor levels were detected by enzyme-linked immunosorbent assay and real-time polymerase chain reaction (RT-PCR). Cell models were first treated with sodium butyrate (4 μmol/mL), followed by LPS (1 μg/mL) half an hour later in GPR43 small interfering RNA (siRNA)-transfected or control RAW264.7 cells. Cell-inflammation status was evaluated through detecting pro-inflammatory-factor expression by RT-PCR and also through checking toll-like receptor 4/nuclear factor-κB (TLR4/NF-κB)-element levels including TLR4, TRAF6, IKKβ, IкBα, phospho-IкBα, p65, and phospho-p65 by Western blot. The interaction between GPR43 and β-arrestin-2 was tested by co-immunoprecipitation.

Results

Sodium butyrate reversed the LPS-induced tissue-morphology changes and high levels of serum alanine aminotransferase, aspartate transaminase, myeloperoxidase, TUNEL, and pro-inflammatory cytokines such as tumor necrosis factor-α and interleukin-6. The ameliorating effect of sodium butyrate was weakened in GPR43-KO mice and GPR43 siRNA RAW264.7 cells, compared with those of GPR43-positive controls. Sodium butyrate downregulated some elements of the TLR4/NF-κB pathway, including phospho-IκBα and phospho-p65, in RAW264.7 cells. Increased interactions between GPR43 and β-arrestin-2, and between β-arrestin-2 and IкBα were observed.

Conclusion

Sodium butyrate significantly attenuated LPS-induced liver injury by reducing the inflammatory response partially via the GPR43/β-arrestin-2/NF-κB signaling pathway.

β-arrestin2 deficiency protects against hepatic fibrosis in mice and prevents synthesis of extracellular matrix

Hepatic fibrosis is a disease of the wound-healing response following chronic liver injury, and activated hepatic stellate cells (HSCs) play a crucial role in the progression of hepatic fibrosis. β-arrestin2 functions as a multiprotein scaffold to coordinate complex signal transduction networks. Although β-arrestin2 transduces diverse signals in cells, little is known about its involvement in the regulation of liver fibrosis. Our current study utilized a porcine serum-induced liver fibrosis model and found increased expression of β-arrestin2 in hepatic tissues with the progression of hepatic fibrosis, which was positively correlated with collagen levels. Furthermore, changes in human fibrotic samples were also observed. We next used β-arrestin2^−/− mice to demonstrate that β-arrestin2 deficiency ameliorates CCl₄-induced liver fibrosis and decreases collagen deposition. The in vitro depletion and overexpression experiments showed that decreased β-arrestin2 inhibited HSCs collagen production and elevated TβRIII expression, thus downregulating the TGF-β1 pathway components Smad2, Smad3 and Akt. These findings suggest that β-arrestin2 deficiency ameliorates liver fibrosis in mice, and β-arrestin2 may be a potential treatment target in hepatic fibrosis.

The peripheral CB1 receptor antagonist JD5037 attenuates liver fibrosis via a CB1 receptor/β-arrestin1/Akt pathway

BACKGROUND AND PURPOSE

Liver fibrosis is a serious cause of morbidity and mortality worldwide and has no adequate treatment. Accumulating evidence suggests that cannabinoid CB₁ receptors regulate a variety of physiological and pathological processes in the liver, and blockage of CB₁ receptor signalling shows promise as a new therapy for several liver diseases. The aim of this study was to investigate the potential therapeutic effects of CB₁ receptors and a peripheral CB₁ receptor antagonist JD5037 in liver fibrogenesis.

EXPERIMENTAL APPROACH

Liver samples from both humans and mouse models were investigated. The peripheral CB₁ receptor antagonist JD5037, β-arr1 wild type (β-arr1-WT) and β-arr1 knockout (β-arr1-KO) littermate models, and primary hepatic stellate cells (HSCs) were also used. The mechanisms underlying CB₁ receptor-regulated HSCs activation in fibrosis and the therapeutic potential of JD5037 were further analysed.

KEY RESULTS

CB₁ receptors were induced in samples from patients with liver fibrosis and from mouse models. These receptors promoted activation of HSCs in liver fibrosis via recruiting β-arrestin1 and Akt signalling, while blockage of CB₁ receptors with JD5037 attenuated CB₁ receptor-regulated HSCs activation and liver fibrosis by suppressing β-arrestin1/Akt signalling.

CONCLUSIONS AND IMPLICATIONS

CB₁ receptors promote the activation of HSCs and liver fibrosis via the β-arrestin1/Akt signalling pathway. The peripheral CB₁ receptor antagonist JD5037 blocked this pathway, the activation of HSCs and liver fibrosis. This compound and the associated pathway may be a novel approach to the treatment of liver fibrosis.

G-Protein-Coupled Receptors in Heart Disease

GPCRs (G-protein [guanine nucleotide-binding protein]-coupled receptors) play a central physiological role in the regulation of cardiac function in both health and disease and thus represent one of the largest class of surface receptors targeted by drugs. Several antagonists of GPCRs, such as βARs (β-adrenergic receptors) and Ang II (angiotensin II) receptors, are now considered standard of therapy for a wide range of cardiovascular disease, such as hypertension, coronary artery disease, and heart failure. Although the mechanism of action for GPCRs was thought to be largely worked out in the 80s and 90s, recent discoveries have brought to the fore new and previously unappreciated mechanisms for GPCR activation and subsequent downstream signaling. In this review, we focus on GPCRs most relevant to the cardiovascular system and discuss traditional components of GPCR signaling and highlight evolving concepts in the field, such as ligand bias, β-arrestin-mediated signaling, and conformational heterogeneity.

Upstream regulators of phosphoinositide 3-kinase and their role in diseases

Phosphoinositide 3-kinase (PI3K), a crucial signaling molecule, is regulated by various upstream regulators. Traditionally, receptor tyrosine kinases and G protein-coupled receptor are regarded as its principle upstream regulators; however, recent reports have indicated that spleen tyrosine kinase, β-arrestin2, Janus kinase, and RAS can also perform this role. Dysregulation of PI3K is common in the progression of various diseases, including, but not limited to, tumors, Alzheimer's disease, Parkinson's disease, rheumatoid arthritis, and acute myelogenous leukemia. The aim of this review is to provide a perspective on PI3K-related diseases examining both the classical and nonclassical upstream regulators of PI3K in detail.

Ginsenoside metabolite compound-K regulates macrophage function through inhibition of β-arrestin2

Ginsenoside metabolite compound-K (C-K), which is an active metabolite of ginsenoside in vivo, can produce anti-inflammatory affects by activating glucocorticoid receptors (GRs) to inhibit the expression of β-arrestin2. Studies have shown that C-K can inhibit the function of immune cells including macrophage polarization and phagocytosis. However, the mechanism by which C-K regulates macrophage polarization is currently unclear. Toll-like receptors (TLRs) are the pattern recognition receptors on the membrane of immune cells, with TLR4 being especially important in polarization of macrophages. The Gαi-mediated activation of nuclear factor-κB (NF-κB) by TLR4 promotes inflammation and phagocytosis in macrophages by increasing the proportion of type I phenotypic macrophages (M1). Whether C-K inhibits the signal transduction of TLR4-Gαi-NF-κB and how that effects macrophage polarization regulation in murine models of RA is not reported. The coupling of G proteins with receptors is regulated by β-arrestin2, but it has been unclear whether C-K modulates the TLR4 interaction with G proteins by inhibiting the expression of β-arrestin2. To explore these questions, the collagen-induced arthritis (CIA) mouse model was employed, and mice were treated with C-K (112 mg/kg/day). The results depict that C-K treatment inhibits macrophage phagocytosis and reduces the proportion of M1. C-K decreases the overexpressed β-arrestin2, Gαi, TLR4 and NF-κB in macrophages of CIA mice, while increasing the expression of Gαs. Furthermore, C-K promotes TLR4-Gαs coupling and inhibits TLR4-Gαi coupling through β-arrestin2 regulation in macrophages, leading to a decrease in the proportion of M1 to M2 macrophages and improved outcomes in CIA mice.

Therapeutic Potential of Targeting ß-Arrestin

ß-arrestins are multifunctional proteins that modulate heptahelical 7 transmembrane receptors, also known as G protein-coupled receptors (GPCRs), a superfamily of receptors that regulate most physiological processes. ß-arrestin modulation of GPCR function includes termination of G protein-dependent signaling, initiation of ß-arrestin-dependent signaling, receptor trafficking to degradative or recycling pathways, receptor transactivation, transcriptional regulation, and localization of second messenger regulators. The pleiotropic influence ß-arrestins exert on these receptors regulates a breadth of physiological functions, and additionally, ß-arrestins are involved in the pathophysiology of numerous and wide-ranging diseases, making them prime therapeutic targets. In this review, we briefly describe the mechanisms by which ß-arrestins regulate GPCR signaling, including the functional cellular mechanisms modulated by ß-arrestins and relate this to observed pathophysiological responses associated with ß-arrestins. We focus on the role for ß-arrestins in transducing cell signaling; a pathway that is complementary to the classical G protein-coupling pathway. The existence of these GPCR dual signaling pathways offers an immense therapeutic opportunity through selective targeting of one signaling pathway over the other. Finally, we will consider several mechanisms by which the potential of dual signaling pathway regulation can be harnessed and the implications for improved disease treatments.

GRKs and β-Arrestins: "Gatekeepers" of Mitochondrial Function in the Failing Heart

Mitochondrial regulation of energy production, calcium homeostasis, and cell death are critical for cardiac function. Accordingly, the structural and functional abnormalities of these organelles (mitochondrial dysfunction) contribute to developing cardiovascular diseases and heart failure. Therefore the preservation of mitochondrial integrity is essential for cardiac cell survival. Mitochondrial function is regulated by several proteins, including GRK2 and β-arrestins which act in a GPCR independent manner to orchestrate intracellular signaling associated with key mitochondrial processes. It is now ascertained that GRK2 is able to recover mitochondrial function in response to insults. β-arrestins affect several intracellular signaling pathways within the cell which in turn are involved in the regulation of mitochondrial function, but a direct regulation of mitochondria needs further investigations. In this review, we discuss the recent acquisitions on the role of GRK2 and β-arrestins in the regulation of mitochondrial function.

Dopamine D2 receptor restricts astrocytic NLRP3 inflammasome activation via enhancing the interaction of β-arrestin2 and NLRP3

Astrocytes are involved in the neuroinflammation of neurodegenerative diseases, such as Parkinson's disease (PD). Among the numerous inflammatory cytokines, interleukin-1β (IL-1β) produced by astrocytic Nod-like receptor protein (NLRP) inflammasome is crucial in the pathogenesis of PD. β-arrestin2-mediated dopamine D2 receptor (Drd2) signal transduction has been regarded as a potential anti-inflammatory target. Our previous study revealed that astrocytic Drd2 suppresses neuroinflammation in the central nervous system. However, the role of Drd2 in astrocytic NLRP3 inflammasome activation and subsequent IL-1β production remains unclear. In the present study, we used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced PD mouse model to investigate whether Drd2 could suppress astrocytic NLRP3 inflammasome activation. We showed that Drd2 agonist inhibited NLRP3 inflammasome activation, evidenced by decreased caspase-1 expression and reduced IL-1β release in the midbrain of wild type mice. The anti-inflammasome effect of Drd2 was abolished in β-arrestin2 knockout and β-arrestin2 small interfering RNA-injected mice, suggesting a critical role of β-arrestin2 in Drd2-regulated NLRP3 inflammasome activation. We also found that Drd2 agonists suppressed the upregulation of caspase-1 and IL-1β expression in primary cultured mouse astrocytes in response to the activation of NLRP3 inflammasome induced by lipopolysaccharide plus adenosine triphosphate. Furthermore, we demonstrated that β-arrestin2 mediated the inhibitory effect of Drd2 on NLRP3 inflammasome activation via interacting with NLRP3 and interfering the inflammasome assembly. Collectively, our study illustrates that astrocytic Drd2 inhibits NLRP3 inflammasome activation through a β-arrestin2-dependent mechanism, and provides a new strategy for treatment of PD.

Emerging Roles of G Protein-Coupled Receptors in Hepatocellular Carcinoma

Among a great variety of cell surface receptors, the largest superfamily is G protein-coupled receptors (GPCRs), also known as seven-transmembrane domain receptors. GPCRs can modulate diverse signal-transduction pathways through G protein-dependent or independent pathways which involve β-arrestins, G protein receptor kinases (GRKs), ion channels, or Src kinases under physiological and pathological conditions. Recent studies have revealed the crucial role of GPCRs in the tumorigenesis and the development of cancer metastasis. We will sum up the functions of GPCRs—particularly those coupled to chemokines, prostaglandin, lysophosphatidic acid, endothelin, catecholamine, and angiotensin—in the proliferation, invasion, metastasis, and angiogenesis of hepatoma cells and the development of hepatocellular carcinoma (HCC) in this review. We also highlight the potential avenues of GPCR-based therapeutics for HCC.

The role and mechanism of β‑arrestins in cancer invasion and metastasis (Review)

β-arrestins are a family of adaptor proteins that regulate the signaling and trafficking of various G protein-coupled receptors (GPCRs). They consist of β-arrestin1 and β-arrestin2 and are considered to be scaffolding proteins. β-arrestins regulate cell proliferation, promote cell invasion and migration, transmit anti-apoptotic survival signals and affect other characteristics of tumors, including tumor growth rate, angiogenesis, drug resistance, invasion and metastatic potential. It has been demonstrated that β-arrestins serve roles in various physiological and pathological processes and exhibit a similar function to GPCRs. β-arrestins serve primary roles in cancer invasion and metastasis via various signaling pathways. The present review assessed the function and mechanism of β-arrestins in cancer invasion and metastasis via multiple signaling pathways, including mitogen-activated protein kinase/extracellular signal regulated kinase, Wnt/β-catenin, nuclear factor-κB and phosphoinositide-3 kinase/Akt.

Macrophages and the Recovery from Acute and Chronic Inflammation

In recent years, researchers have devoted much attention to the diverse roles of macrophages and their contributions to tissue development, wound healing, and angiogenesis. What should not be lost in the discussions regarding the diverse biology of these cells is that when perturbed, macrophages are the primary contributors to potentially pathological inflammatory processes. Macrophages stand poised to rapidly produce large amounts of inflammatory cytokines in response to danger signals. The production of these cytokines can initiate a cascade of inflammatory mediator release that can lead to wholesale tissue destruction. The destructive inflammatory capability of macrophages is amplified by exposure to exogenous interferon-γ, which prolongs and heightens inflammatory responses. In simple terms, macrophages can thus be viewed as incendiary devices with hair triggers waiting to detonate. We have begun to ask questions about how these cells can be regulated to mitigate the collateral destruction associated with macrophage activation.